1. Field of the Invention
The present invention relates to the field of scanning. More specifically, the present invention relates to a method of scanning an image, and a scanning device arranged to perform the method of scanning In a scanning device of the present invention, an optical sensor is provided comprising sensor elements for each of a number of basic colors to generate pixel values of a scanned image. Each pixel value represents an optical density of a pixel of the image. In this way, a monochrome, i.e. grey scale or black and white (B/W) image of the original is obtained using a color scanner.
2. Background of the Invention
A scanner usually has a light source emitting white light. In reduction type scanners, an optical sensor with one or more linear CCD (Charge Coupled Device) arrays and a lens are arranged to focus lines of an original onto the CCD line arrays. In contact type scanners, lines of the original are captured onto the CCD line arrays through a lens array.
In a color scanner, for example, there are at least three CCD line arrays having color filters for red (R), green (G), and blue (B) light, respectively, thus forming sensor elements for the basic colors of red, green, and blue light. Typically, the sensor elements for different colors have different light sensitivities. When a grey scale image is to be obtained from a color original, the original is scanned, and the signals from the sensor elements for red, green, and blue light are mathematically mixed or processed in order to calculate the grey scale intensities from the RGB values of each pixel of the image. Hence, the grey scale level of a pixel is a weighted average of the RGB levels, wherein the weight factors are defined such that the hue and saturation information is eliminated. A usual RGB weighing scheme is: grey scale level=0.3·R+0.6·G+0.1·B.
The scan speed of a reduction type scanner is limited by the maximum clock speed of the CCD. This is a serious limitation in view of a demand for high scan resolutions to be obtained in a shortest possible time.
Reference WO 2007/062679 A1 discloses a high speed scanner and method of scanning an image on an original, wherein an optical sensor comprising sensor elements for each of three basic colors is used to generate pixel values of the scanned image. Each pixel value represents an optical density of a pixel of the image. Each pixel value is generated mainly on the basis of a signal of only one of the sensor elements. Each of the sensor elements for each of the basic colors is used to generate pixel values representing substantially different parts of the image. Accordingly, the amount of pixels to be scanned is divided among the sensor elements for the different basic colors, and the scan speed can be increased up to three times compared to a conventional scan mode wherein each pixel of the original is scanned by three sensor elements. Thus, a color scanner can be used with a high scan speed when only a grey scale or B/W image is needed.
In the scanner and method according to the WO 2007/062679 A1 reference, artifacts due to a repeat frequency of a pattern in the scanned image at one third of the output frequency in a transport direction of the original are produced, requiring a subsequent filtering process to suppress such artifacts.
The image on the original may be a grey scale or B/W image, or may be a color image. In a color image, some colors may not be recognized by the sensor element scanning the colors. For example, a green pixel on the original will not be recognized by a sensor element for color red and a sensor element for color blue. Accordingly, when scanning a green area on the original, artifacts are produced having a spatial frequency corresponding to a repeat length in the scan direction of the image.